Re: [PATCH] Significant performace improvements on reiserfs systems

[Andrew Morton <akpm@zip.com.au>]

george anzinger wrote:
> 
> ...
> Actually, I rather think that the problem is lock granularity.  These
> issues are present in the SMP systems as well.  A good solution would be
> one that shortened the spinlock time.  No horrid preempt code, just
> tight fast code.
> 

This may not be practical.

Take, for example, zap_page_range().   It simply has a lot
of work to do, and it does it inside a spinlock.  By doing
it in a tight loop, it's optimal.

There is no way to speed this function up by two or three orders
of magnitude.  (Well, there is: don't take the lock at all if
the mm isn't shared, but this is merely an example.  There are
other instances).

It seems that for a preemptive kernel to be successful, we need
to globally alter the kernel so that it never holds locks for
more than 500 microseconds.  Which is what the conditional_schedule()
(aka cooperative multitasking :)) patches do.

It seems that there are no magic bullets, and low latency will
forever have a global impact on kernel design, unless a way is
found to reschedule with locks held.  I recall that a large
part of the MontaVista patch involved turning spinlocks into
semaphores, yes?  That would seem to be the way to go.